Conventional organic brake linings are made up of a mixture of asbestos fibers and modifiers held together in a fixed matrix created by a phenolic resin. Phenolic resins are produced through the condensation of a phenol with formaldehyde in the presence of a catalysis. The resin is then polymerized at a temperature of between 150.degree.-250.degree. C. to develop a water insoluble and infusible mass. Phenol is made from cumene hydroperoxide which is derived from acid-catalyzed alkalization of benzene with propylene. Unfortunately, the supply of benzene has been reduced because of changes in the fractionization or cracking process of petroleum required to produce lead-free or low lead gasoline needed to operate automotive vehicles now being manufactured. Because of this reduction in the supply of benzene, it is necessary that an extender or substitute for phenolic resin be found in order to maintain economic stability of friction materials.
Through experimentation it has been determined that the oily liquid located between the double shell of the cashew nut is a natural phenolic substance. The nutshell liquid upon being heated with a hardening agent, such as hexamethylenetetramine or formaldehyde, polymerizes into a water insoluble mass. However, since such cashew nutshell liquid is presently imported from South America and Asia, and since spiraling transportation costs have caused the price of such cashew liquid to increase substantially, it is no longer economical to use only cashew liquid and therefor an extender is needed in the production of brake linings.
In view of the foregoing it was suggested that lignin or a lignin derivative be combined with a phenolic resin to produce a fixed matrix in friction lining inasmuch as lignin was known to be a cementing agent which binds the matrix of cellulose fibers in a woody structure. Furthermore, sources of lignin are so abundant that much of it is now burned to provide thermal energy.
The most accessible source of lignin is a by-product from the pulp and paper industry which alone is capable of supplying unlimited quantities of lignin. This by-product is produced by the paper industry in the following manner: Wood chips are utilized to produce a pulp from which paper is produced. This pulp is processed through the action of either a sulfide or a sulfate solution on the wood chips.
In the sulfide pulp process, the wood chips are placed in a digester where they are cooked in a diluted solution of calcium, magnesium , sodium or ammonium bisulfite (calcium bisulfite is most often used), free sulfur dioxide gas, and water under pressure. The sulfur dioxide gas and water form sulfurous acid which sulfonates the lignin and renders it water soluble to produce a sulfite liquor. The sulfite liquor is then removed leaving the cellulose fiber, from which paper is made, as a residue. When the sulfite liquor is treated with calcium hydroxide and heated, a lignosulfonate precipitate is formed. Evaporation of water yields a brown colored fine solid commonly identified as calcium lignosulfonate.
In the sulfate or kraft process, the wood chips are placed in a digester where they are cooked in a solution of sodium hydroxide and sodium sulfide. The sulfide ions react with the lignin in the wood chips to form sodium phenolate salts which are soluble in the cooking liquor. Because the cooking time in the sulfate process is less than in the sulfite process there is less time for the sulfide ions to breakdown the carbohydrate portion of the cellulose material. Thus, sulfate processed pulp is stronger than the pulp produced by the sulfite process and for this reason the sulfate process is normally employed in the paper making industry. Because of the cost of sodium hydroxide and sodium sulfide, the cooking liquor normally is evaporated to reclaim the sodium and sulfur. However, if a filtering process is used in place of the evaporation, after the cooking liquor is neutralized, a kraft or alkali lignin is produced. The process for the production of this kraft lignin is fully disclosed in U.S. Pat. No. 2,997,466 and U.S. Pat. No. 3,048,596. The resulting kraft lignin will be dark brown fine solid.
In this specification the term lignin is used interchangeably with lignosulfonate and kraft lignin. In general, most lignins have the following characteristics: they are brown in color, have a density of between 1.3-1.4 and an index of refraction of about 1.6. However, lignosulfonates are soluble in water and insoluble in mineral oil and hydrocarbons, while kraft lignins are soluble in aqueous alkaline solutions, and amines or oxygenated organic compounds requiring different carriers to bring about uniform mixing. Additionally, while lignosulfonates do not have any visible crystalline form, x-ray diffraction studies indicate that lignosulfonate is an amorphous polymer similar to the kraft lignin. It has been shown that the kraft lignin has a sintering point and tends to flow at elevated temperatures similar to phenolics as contrasted to lignosulfonates which do not melt but do experience some softening before charring.
In U.S. Pat. No. 2,285,801 it is disclosed how lignin could be combined with phenol in the presence of urea to form a binder for holding mineral wool in filters and batting used to insulate houses. However, such a combination presented severe limitations since this combination was never charred to a solid mass and as a result, when less than 20% phenolic was present in the mixture a strong bond could not be obtained and with more than 50% of lignin the stability of the matrix was impared in ambient temperatures.